"Okoi" is the protagonist of the "Fishy Tales of the Nekomata" downloadable content in Muramasa Rebirth. In reality, "Okoi" is actually Miike, a cat who belonged to the real Okoi before she died and assumed the girl's form to continue her owner's duty before avenging her as a nekomata.
Originally a cat named Miike, she accompanied her owner Okoi and her brother in delivering a prized tea cup to the shogun. However, Miike is forced to watch the siblings murdered by assassins sent by Shinzaemon Shigematsu, drinking the deceased Okoi's spilled blood to become a Nekomata that assumes her owner's appearance. From there, Miike kills the assassins and delivers the tea cup to honor the original Okai's dying wish. However, learning that Okoi's father committed suicide, Miike vowed to avenge the death of her family by turning to the Tanuki Danzaburo to obtain greater power. However, even after killing Shinzaemon and his employer Netsuzo Wakamiya, Miike was not satisfied as she extends her wrath to the entire Wakamiya clan. Unfortunately, this proves to Miike's undoing when her tails are chopped off by Wakamiya clan's hired help Jinkuro. Soon after, rendered a normal cat, Miike took residence in a shrine for years as the priest's cat Suzu while secretly hosting a social gathering for cats on a nightly basis.
In an alternate ending, Miike uses her powers to devour travelers in an attempt to satiate her new never-ending hunger. Her latest victim, a young monk who was running errands, manages to escape by feigning sleep (after unintentionally wetting the futon for being scared) and escaping before she could eat him. The young monk runs all the way back to his temple, where he seeks refuge from the High priest living there. Miike demands that the High Priest hands over the young monk, or she will devour him instead. The High priest offers Miike a test where he would offer himself and the young monk if she would manage to pass. Miike, who was eager to prove her abilities, accepts and turns herself into a bean in response to the High Priest's suggestion. However the High Priest grabs Miike in her new form, stuffs her in a rice cake, and eats her. But as the High Priest tells the young monk there is no such things as monsters, with a cat closely resembling Miike nearby, it turned to all be the boy's imagination in response to the hysteria of events occurring at the time.
Unlike the main protagonists who use demon blades, Okoi uses her claws and special abilities to rain down death and destruction. She is capable of taking both human form and Nekomata form, the latter of which she can transform into a monstrous version of.
Okoi is shown to be quick, attacking with multiple claw strikes. She is capable of throwing blue fireballs that appear to be similar to foxfire. In her natural cat form, Miike can fly around by using her tails like a helicopter or use transformation magic. She is also seen performing a dance that causes dozens of cats to rain from the sky. 
- Miike's name is based derived from the Calico cat breed (三毛猫 Mikeneko).
- In the alternate ending of the DLC, the confrontation between Miike and the High Priest references the final scene of the Japanese fairytale Sanmai no Ofuda ( 三枚のお札, "The Three Charms"), where an old priest tricks an evil mountain witch into making herself as small as a bean, then prompty stuffs her into a rice cake and eats it.